The printing and graphic arts industries require printing plates coated with substances which are suitable for direct digital imaging by laser. The graphic image stored and created on computers can be output to a near infrared laser digital imaging device which will "draw" the image on the printing plate coating by eliciting a localized transformation of the coating. This method has the distinct advantage of not requiring a wet development step. The printing plate can act as a positive or a negative depending on the laser effect on the coating.
In the case of the present invention, the coating is oleophilic (attracts ink) and the undercoating is hydrophilic. Depending on where the laser image is drawn on the coating, the printing plate may be made to act as a positive or negative plate. Indeed, exposure to laser radiation renders the coating locally hydrophilic which causes it to wash away with water based inks and fountain solutions.
Various near infrared absorption polymers have been proposed in the prior art. However, these polymers face the various drawbacks outlined below.
U.S. Pat. No. 5,362,812 discloses reactive polymeric dyes comprising an azlactone moiety for use in photoresist systems and color proofing media. These materials contain visible light absorption chromophoric moiety and acrylate functional groups, which undergo cross-linking reactions via free radical initiating upon exposure to ultraviolet light. However, these materials cannot be used for lithographic printing plate due to an absence of absorption in the near infrared region.
U.S. Pat. No. 4,666,819 and 4,680,375 teach the preparation of polymers containing cyanine dyes in the polymer backbone for optical recording. These materials are sufficiently stable so as not to be chemically changed upon exposure to near infrared laser light and are not useful in lithographic printing.
U.S. Pat. No. 5,824,768 also teaches to the preparation of polymeric dyes for optical recording. More particularly these dyes are used as coatings in recordable CDs. These materials do not undergo chemical changes upon exposure to near infrared laser light and are not useful.
EP 0652483A1 teaches the preparation of lithographic printing plates, which by virtue of a coating composition can be imaged by near infrared laser light and which do not require wet chemical development. The coating composition contains near infrared absorption dyes, thermal acid generators and polymers having pendant hydrophobic groups such as t-alkyl carboxylates, t-alkyl carbonates, benzyl carboxylates and alkoxyalkyl esters. Upon exposure to near infrared laser light, the exposed area becomes hydrophilic, therefore repelling water based inks. The unexposed areas remain hydrophobic and become the image areas. This type of coating composition however requires high laser power to image and is sensitive to handling.
U.S. Pat. No. 5,569,573 also teaches the preparation of lithographic printing plates, which can be imaged with near infrared laser light and do not require wet chemical development. The laser imaging layer comprises microencapsulated oleophilic materials in hydrophilic binder resins capable of forming a three dimensional network with the microcapsulated oleophilic materials upon exposure to near infrared laser light to becomes image area. This type of coating is difficult to prepare and requires high laser power to achieve imaging.
EP 0 770 495 A1 teaches to prepare lithographic printing plates, which can be imaged with near infrared laser light. The unexposed areas are removed on press with ink and fountain solution. The imaging layer comprises near infrared absorption materials, polymer binders and thermoplastic particles capable of coalescing under heat. This type of coating offers poor substrate adhesion and consequently fail to provide sufficient run length on press.
Thus there remains a need for new polymer coating compositions which overcome the drawbacks of the prior art coatings. The goals are to achieve coating compositions combining the advantages of long-life printing plates, absence of phase separation of the different ingredients in the coating formulation, easily manufactured and inexpensive coating formulations, precisely imagable coatings providing high image resolutions.